The 1980s are meant to be music's tacky decade: all fun and fluoro colours, but a little, well, daft. That argument can extend to Prince, who often took to the stage in a zebra G-string and gold hoop earrings. Musically, however, he was always more sophisticated – nowhere more so than on the double album Sign "O" the Times, which is 25 years old this year. Prince's sexuality is on display in If I Was Your Girlfriend (is it sung by a man or a woman?); his bombast on The Cross, a guitar-heavy love letter to Jesus Christ. But no song illustrates his genius better than its title track – his only overtly political song, touching on everything from Aids to inequality ("Sister killed her baby 'cuz she couldn't afford to feed it / And we're sending people to the moon"). Sign "O" the Times may well be pop's last great concept album. It's certainly one of the few to give the genre a good name. And its creator remains one of the greatest talents in pop.